1. Field of the Invention
The present invention relates to a chemical vapor deposition apparatus in which a thin film is formed according to the chemical vapor deposition method by using plasma energy and reaction gas.
2. Prior Art
Conventionally, there is a well-known plasma enhanced chemical vapor deposition apparatus (hereinafter, simply referred to as PCVD apparatus), which has a waveguide 1, a reaction tube 2, and a substrate holder 3, as shown in FIG. 14. Herein, the waveguide 1 is vertical, and the reaction tube 2 is provided through the waveguide 1 in such a manner that the reaction tube 2 is positioned against the waveguide 1 in the vertical direction. In addition, the substrate holder 3 is provided at a location where waveguide 1 and reaction tube 2 cross.
In this PCVD apparatus, a substrate 4 is placed on the substrate holder 3, and the reaction gas is supplied to the reaction tube 2 from a gas supply source. In addition, microwaves are introduced into the waveguide 1 from a suitable oscillator so that they propagate through the waveguide 1, whereby plasma is generated around the substrate 4. As a result, the reaction gas around the substrate 4 is decomposed into its components by effect of the plasma generated around the substrate 4, so that the components thus decomposed are deposited on the surface of the substrate 4. In this way, a thin film having the desired composition is formed on the surface of the substrate 4.
As another example of the conventional PCVD apparatus, there is a known apparatus in which the horizontal reaction tube 2 is inserted through the vertical waveguide 1, and the substrate holder 3 is provided in a portion of the waveguide 1 at which the reaction tube 2 is crossed as shown in FIG. 15.
In the case of the conventional PCVD apparatuses described above, wherein the substrate 4 is shaped three-dimensionally like a dome, a strong plasma 5 is generated over the substrate 4, as shown in FIG. 16, so that the reaction gas around the circumference of the substrate 4 is decomposed by the plasma and the components thus decomposed are deposited on the surface of the substrate 4. As a result, the thin film 6 is formed on the peripheral surface of the substrate 4 as shown in FIG. 17.
However, in the case where the thin film is formed by the conventional PCVD apparatus described above, the thickness of the thin film increases toward the upper portion of the substrate 4, while the thickness decreases toward the lower portion of the substrate 4. In an extreme case, the ratio of the thick portion to the thin portion may be as high as 4:1.
Thus, specific research has occurred to determine the reason why such thickness differences occur. The results indicate that the relative position relationship between the strong plasma (the strongest part of the plasma) and the substrate produces important effects on the manner of formation of the thin film. More specifically, in the case where the thin film is deposited on a substrate which is shaped three-dimensionally like a dome, the interaction of the plasma impact becomes weaker as the distance between the substrate and the strong plasma becomes larger, while the interaction of the electron impact becomes stronger as the distance from the substrate to the strong plasma becomes smaller. Therefore, the upper portion of the substrate will be at a high temperature, while the lower portion thereof will be at a low temperature, so that the temperature distribution is nonuniform. As a result, it seems that the thickness of the thin film varies because of the above phenomenon.
Furthermore, in the conventional PCVD apparatuses shown in FIGS. 14 find 15, the reaction tube 2 is designed to pass through the waveguide 1 so that the reaction tube 2 cannot be formed largely because leakage of electromagnetic waves generated through the reaction tube 2 must be prevented. Thus, there is a problem in that a large substrate cannot be used.